Semiconductor wafer coat layers and methods therefor

ABSTRACT

Formulations and processes for forming wafer coat layers are disclosed. In one embodiment, an organic surface protectant is incorporated into a wafer coat formulation deposited onto a semiconductor wafer prior to the laser scribe operation. Upon removal of the wafer coat layer, the organic surface protectant remains on the bumps and thereby prevents oxidation of the bumps between die prep and chip and attach. In an alternative embodiment, an ultraviolet light absorber is added to the wafer coat formulation to enhance the wafer coat layer&#39;s energy absorption and thereby improve the laser&#39;s ability to ablate the wafer coat layer. In an alternative embodiment, a conformal wafer coat layer is deposited on the wafer and die bumps, thereby reducing wafer coat layer thickness variations that can impact the laser scribing ability.

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to semiconductorprocessing and more specifically to forming wafer coat layers onsemiconductor substrates.

BACKGROUND OF THE INVENTION

With a need to incorporate low dielectric constant (low-k) interlayerdielectrics (ILDs) into semiconductor technology, has come therealization that low-k ILDs may not seamlessly integrate into existingsemiconductor process flows. One place where this is evident is at theback-end saw singulation process. This is because low-k materials aremechanically weaker than conventional silicon dioxide. Consequently,sawing can damage the low-k ILD and adjacent circuitry and impact theyield and reliability of the semiconductor device. To overcome this,some manufacturers use lasers to first scribe through the various layersformed over the semiconductor wafer (including the low-k ILD) and thenuse the saw to cut through the bulk of the semiconductor wafer, therebysingulating the wafer with a two-step process.

The use of lasers, however, is not without its own set of manufacturingissues. For example, as the laser scribes the wafer it can produce acloud of debris, and particles from the debris can deposit on exposedwafer surfaces. An illustration of this is shown in the cross-sectionalview of the semiconductor wafer 100 in FIG. 1. Unless removed, thedebris can impact the yield and reliability of singulated semiconductordevices during/after subsequent packaging.

As shown in FIG. 1, a laser beam 108 ablates and thereby removes fromthe scribe line region 110 the various device layers 104 formed oversemiconductor substrate 102. As a result, debris 112 generated by thelaser can deposit onto bumps 106. The debris 112 is problematic becauseto the extent it is not removed, it can produce non-wetting of thesolder bumps during the chip-attach operation. This can result inelectrical opens between semiconductor devices and their correspondingpackaging substrates.

Shown in FIG. 2A is a cross-sectional view of a semiconductor wafer 200Athat has been prepared for laser scribe. Here, the debris 112 problem(shown in FIG. 1) has been addressed by depositing a wafer coat layer207 over the bumps 206 before scribing the wafer 200A (for the purposeof this specification, a wafer coat layer is a layer formed over bumpson a wafer, so as to protect them during the laser scribe operation).So, as shown in FIG. 2B, instead of depositing onto the bumps 206, thelaser-generated debris 212 deposits on the wafer coat layer 207 where itcan be rinsed-off later during the wafer coat layer removal process. Asshown in FIG. 3, after the wafer coat layer and debris 212 (shown inFIG. 2B) have been removed, a scribe line 210 has been formed throughthe various device layers 204 of the semiconductor substrate 300 alongthe wafer's street regions. The scribe line exposes the underlying bulksemiconductor substrate 202 and the bumps 206 have minimal residualdebris on them.

However, the wafer coat layer 207 too present a number of integrationchallenges. For example, the added wafer coat layer increasesmanufacturing cycle time and can cause materials interactions, which inturn can result in increased oxidation of the die bumps and affect dieattach interconnectivity. Also, to the extent that the wafer coat layeris transparent to the laser, it must be removed via some other mechanismbesides ablation. And, to the extent that the wafer coat layer 207optically refracts, diffracts, and/or scatter the laser's beam, it caninterfere with the laser's ability to ablate the various underlyingdevice layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a portion of asemiconductor wafer being scribed using a laser.

FIG. 2A illustrates a cross-sectional view of a portion of asemiconductor wafer having a wafer coat layer.

FIG. 2B illustrates a cross-sectional view of semiconductor wafer ofFIG. 2A while being scribed using a laser.

FIG. 3 illustrates a cross-sectional view of the semiconductor wafer ofFIG. 2B after scribing the semiconductor wafer and removing the wafercoat layer.

FIG. 4 illustrates a cross-sectional view of a portion of asemiconductor wafer having a bump oxidation prevention layer formed inaccordance with an embodiment of the present invention.

FIG. 5 illustrates a cross-sectional view showing laser scribing of aportion of a semiconductor wafer having a conventional wafer coat layer.

FIG. 6 illustrates a cross-sectional view showing laser scribing of aportion of a semiconductor wafer having a wafer coat layer formed inaccordance with an embodiment of the present invention.

FIG. 7 illustrates a cross-sectional view showing laser scribing of aportion of a semiconductor wafer having a wafer coat layer formed inaccordance with an alternative embodiment of the present invention.

FIG. 8 illustrates a cross-sectional view showing a portion of asemiconductor wafer having a non-conformal wafer coat layer formedthereon.

It will be appreciated that for simplicity and clarity of illustration,elements in the drawings have not necessarily been drawn to scale. Forexample, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals have been repeated among the drawings toindicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, methods for improving processesrelated to semiconductor substrate laser scribing are disclosed.Reference is made to the accompanying drawings within which are shown,by way of illustration, specific embodiments by which the presentinvention may be practiced. In other instances, well known features maybe omitted or simplified in order not to obscure embodiments of thepresent invention. It is to be understood that other embodiments mayexist and that other structural changes may be made without departingfrom the scope and spirit of the present invention.

The terms on, above, below, and adjacent as used herein to refer to theposition of one layer or element relative to other layers or elements.As such, a first element disposed on, above, or below a second elementmay be directly in contact with the second element or it may include oneor more intervening elements. In addition, a first element disposed nextto or adjacent a second element may be directly in contact with thesecond element or it may include one or more intervening elements.

In one embodiment, an organic surface protectant (OSP) is incorporatedinto the wafer coat layer's formulation. After laser scribe andsubsequent removal of the wafer coat layer, the OSP component continuesto protect die bumps by preventing oxidation during the time between dieprep and chip-attach. In a second embodiment, an ultraviolet absorber isadded to the wafer coat formulation to make it less transparent to thelaser's energy. In this way, the laser can directly ablate the wafercoat layer and not rely on the superheating of underlying films tothermally degrade and remove wafer coat layer material from the scribeline regions. In a third embodiment, a conformal wafer coat layer isdeposited using vacuum deposition process. Increased conformalitytranslates to a reduction in overall wafer coat thickness variation,which can reduce the refraction, diffraction, and scattering effectsthat the wafer coat layer can have on the laser scribing process. Theseembodiments and variations thereof will be further discussed and may bebetter understood with respect to FIGS. 4-8 discussed below.

In accordance with one embodiment (discussed with respect to FIGS. 2A,2B, and 4), the wafer coat layer is used as a vehicle to apply an OSPlayer to the wafer bumps (i.e., the OSP is included in the wafer coatformulation as an OSP additive). Then when the wafer coat layer isremoved, the OSP remains as a layer on the bumps. Incorporating the OSPadditive into the wafer coat formulation extends the functionality ofthe wafer coat step, reduces cycle time by eliminating the need toseparately apply the OSP layer, and eliminates the need to useadditional processing equipment and floor space. The OSP layer canreduce the amount of die bump oxidation formed between the wafer-coatand chip-attach operations and thereby reduces the level of defectsrelated to die-attach fluxing, as explained below.

Bump oxidation is inherent during the normal assembly process.Typically, this oxide is cleaned from the bump surfaces prior to thechip-attach operation by incorporating a fluxing step. The chip-attachfluxing step enhances the solder bump's wetting ability and therebybonds (and electrically attaches) the die and package substrate togetherduring the chip-attach operation. Wetting ability, as known to one ofordinary skill, refers to the ability of the bump, while in a moltenmetal state, to wet the substrate surface under a given set ofconditions. This is an important performance parameter because itdirectly affects the integrity of bump's interconnection with thepackaging substrate. To the extent that heavy oxidation of the bumpsexists, outgassing caused by reduction of metal oxides (i.e., the bumpoxidation) during the chip-attach fluxing step can produce voids in thebumps and/or underfill material that can affect die yields and/orreliability.

Bumps can be protected against oxidation by forming an organic surfaceprotectant (OSP) over them. In accordance with this embodiment, the OSPlayer disclosed herein is incorporated into an existing processing step(i.e., the wafer coat step). Decreasing the amount of oxide formed onthe bumps also decreases the amount of flux necessary to remove bumpoxidation. This can reduce outgassing and the formation of voids in thebumps and underfill. Bump voids can impact the bumps ability to conductcurrent, and underfill voids can impact the die's ability to adhere tothe packaging substrate. Both of which can impact the packaged devicesyield and reliability.

Referring to FIGS. 2A, 2B, and 4, a process incorporating anOSP-containing wafer coat layer will be described. It should be apparentto one of ordinary skill that one advantage of this embodiment is itsability to easily be incorporated into existing process flows. As such,the wafer coat layer can be formed over semiconductor device layers andbumps, similar to that shown in FIG. 2A. Unlike the prior art however,the wafer coat layer has an OSP additive incorporated into the wafercoat layer formulation. In accordance with one embodiment, the wafercoat layer can be a conventional water soluble BGA attach flux, such asthe Sparkle Flux FTW-385 manufactured by Senju Metal Industry Co., Ltd.and the OSP can include any number OSP chemistries, such as for examplebenzotriazol, benzimidazol, imidazol, and the like. The OSP additive canbe added to the flux at or prior to its point of use or it can be addedby the supplier of the wafer coat material.

The combination OSP/wafer coat formulation can then be applied on thewafer using a conventional wafer coat process, such as for example usinga blanket spray-on process, a spin-on process, dipping, screen printing,or the like. After the OSP-containing wafer coat layer has beendeposited, the wafer can be laser scribed to ablate/remove the variousdevice layers from over the substrate, similar to that shown in FIG. 2B.Debris that is generated deposits onto to the wafer coat layer. Then, asshown in the cross-sectional view of the wafer 400 of FIG. 4, the wafercoat layer is removed. Removal can be accomplished using a conventionalwafer coat clean. In embodiments, where the wafer coat layer is watersoluble BGA attach flux, the wafer coat layer can be removed usingwater, a water-based solution, or the like. As shown in FIG. 4, unlikethe prior art which completely removes the wafer coat, the OSP layercomponent 402 remains over the bumps 206 where it can limit the amountof subsequent bump oxidation prior to chip-attach. The wafer canthereafter be singulated into individual die using a conventionalmechanical sawing process. Removal of the wafer coat layer can beremoved before, during, or after the wafer sawing process.

The OSP-containing singulated die can then be processed through anynumber of processing steps intermediate to the chip-attach operationwith reduced concerns of encountering oxidation-related problems. Forexample, to the extent that the OSP-containing singulated die is to beprocessed through a chip-attach module that includes a capillaryunderfill (CUF) process flow, the die can thereafter be attached to apackaging substrate using a conventional chip-attach flux and reflowoperation. In accordance with one embodiment, during the reflow toattach the die and package substrate, the flux is activated and the OSPand oxide that have formed on the bumps are thereby removed. Afterreflow, chip-attach flux residues can be removed using a conventionalde-flux process (e.g., hot water). The attached die/package substratecan then continue on to the underfill process module, where underfillmaterial is introduced between the die and the packaging substrate via aconventional capillary underfill process. In accordance with embodimentsthat use this type of process flow, the OSP layer, by reducing theamount of oxide formed on the bumps, can decrease the amount ofchip-attach flux necessary to remove bump oxidation and/or the amount ofoutgassing that can occur during die attach reflow. This can reduce theoccurrence of voids in the bumps and thereby improve their mechanicalintegrity and ability to pass current.

To the extent that the OSP-containing singulated die is to be processthrough a chip-attach module that includes a no-flow (NUF) underfillprocess, the OSP-containing singulated die can then be attached to apackaging substrate by heating the packaging substrate, dispensing theNUF material on it, positioning the die on/in the NUF and then using athermal-compression bonding process to reflow and attach the die to thepackaging substrate. Unlike the capillary underfill process, underfillin the NUF process is dispensed onto the substrate prior to the diebeing bonded to the packaging substrate. In the NUF process, the flux isembedded in the NUF formulation. So, during reflow, the chip-attach fluxactivates and removes the oxide and OSP thereby allowing the die bumpsand substrate solder bumps to reflow and form interconnected joints.After reflow, further cure of underfill can be performed in a post cureoven. In accordance with embodiments that use this type of process flow,the OSP layer, by reducing the amount of oxide formed on the bumps,decreases the amount of outgassing that can occur during die attachreflow. This can reduce the occurrence of voids both in the bumps and inthe underfill. To the extent that voids in the bumps are reduced, theirmechanical integrity and ability to pass current correspondinglyincreases. To the extent that voids in the underfill are reduced, themechanical integrity of the bond between the die and packaging substratecorrespondingly increases.

In accordance with an alternative embodiment discussed with respect toFIGS. 5 and 6, a scribing laser's ability to ablate underlying devicelayers is improved by adding an ultraviolet (UV) absorber to the wafercoat formulation. The UV absorber can enhance the wafer coat layer'sability to absorb laser energy thereby facilitating its ablation. Inthis way, the wafer coat layer, which is substantially transparent tothe laser's UV energy, can be removed or degraded (i.e. the physicalintegrity of the wafer coat layer can be degraded) before ablating theunderlying device layers. To the extent that the wafer coat layer isremoved via ablation, the laser can then more efficiently ablate theunderlying device layers. To the extent that the wafer coat layer can bedegraded, explosive removal of the wafer coat layer incident tounderlying device layer ablation is accomplished more easily. In otherwords, in accordance with this embodiment, a process is disclosedwhereby the wafer coat layer burns away/degrades from the top downduring the laser scribe operation rather than being mechanicallydisplaced (e.g., explodes away) from the bottom up.

Shown in FIG. 5 is a representative cross-sectional view of the laserscribing of a wafer 500 having a conventional wafer coat layer 507. FIG.5 includes a semiconductor substrate 502 over which semiconductor devicelayers 504, conductive bumps 506, and the conventional wafer coat layer507 are formed. As indicated in FIG. 5 by the dashed line portions ofthe laser beam 508, the wafer coat layer 507 is substantiallytransparent to the laser's ultraviolet beam energy. Consequently, thelaser beam 508 passes through the wafer coat layer 507 and then uponreaching the underlying device layer 504, ablates it. Consequently, thewafer coat layer 507 is not removed by the laser beam 508. Instead, thewafer coat layer's removal occurs as a result of superheating thoseportions 510 of the device layers 504 and then using superheated gassesto thermally degrade and eventually explode away (not shown) portions ofthe wafer coat layer overlying the scribe line street regions.

To the extent that anything interferes with the laser's ability toexplode away portions of the wafer coat layer over the scribe line,residual device layer scribe line material can be left intact, in whichcase defects in the form of unablated scribe line material can result.These defects can cause damage to device layers in adjacentsemiconductor die during the wafer sawing process. Defect sources may,among other things, be the result of thickness or composition variationsin the wafer coat layer. In which case those portions of the wafer coatlayer that are thicker or more resistant to physical (i.e. explosive)removal have the potential to not be removed and thereby result in theformation of defects.

Thickness variations can be attributed to, among other things, pooruniformity of the wafer coat layer spray coating; the relative proximityand spatial variation of wafer bumps or test structures near the scribestreet; and/or small scale de-wetting and localized collection/poolingof the wafer coat layer as it adheres to structures having differentcompositions (e.g., surface tensions effects associated with the waferbumps, passivation layer and/or recesses in the scribe street area).Composition variations in the wafer coat layer can be the result of,among other things, localized changes in the integrity of the wafer coatlayer as a result interactions between the wafer coat layer withunderlying passivation layer materials, bump metallization, etc.

Regardless of which cause is most responsible for generating thesedefects, modifying the wafer coat layer so it ablates from the top-downrather than from the bottom-up has the potential to reduce/eliminatedefects caused by these problems. An example of the mechanics oftop-down ablation using a wafer coat layer 608 modified in accordancewith an embodiment of the present invention is illustrated in FIG. 6.Shown in FIG. 6 is a cross-section of a portion of a wafer 600. Thewafer 600 includes a semiconductor substrate 602 over whichsemiconductor device layers 604, conductive bumps 606, and a modifiedwafer coat layer 607 are formed. As shown in FIG. 6, unlike theconventional wafer coat layer of FIG. 5, the wafer coat layer 607 isformulated in accordance with an embodiment of the present invention, soit can absorb UV energy from laser beam 608. As a result, portions 610of the wafer coat layer 607 are removed via ablation. So here, as aresult of including UV absorbing additive(s) into the wafer coat layerformulation, the mechanics of the wafer coat layer's removal duringscribing changes from a physical explosive process to direct (orindirect) ablation. In this way, those portions of the wafer coat layeroverlying the wafer street region are ablated prior to or concomitantlyalong with the underlying device layers. To the extent that the wafercoat layer is ablated prior to ablating the underlying device layer,occurrences of residual ablated material being trapped under unexplodedwafer coat layer portions can be significantly reduced. Alternatively,the UV absorber can be included in the wafer coat layer 607 in an amountthat increases absorption of the laser's UV radiation without causingsubstantial ablation of it. This can weaken the integrity of the wafercoat layer so when ablation of the underlying device layers does occur,removal of the wafer coat layer incident to the underlying ablationoccurs much more easily than it otherwise could.

The wafer coat formulation can be made UV absorbing by including organicadditives capable of absorbing radiation at the laser's operatingfrequency. In embodiments where the laser's frequency is in the UV-Aportion of the electromagnetic spectrum, these additives can includemolecules generally described as benzophenones, diphenyl acrylates,cinnamates and sterically hindered amines. Selection of the appropriateadditive should take into account compatibility with the base wafer coatlayer material (i.e., the UV absorber should be compatible with thewafer coat material). The degree of solubility of the additive candepend upon the molecular group(s) attached to it. As such, the additivecan be made more or less soluble relative to the base wafer coat layerby changing the type and nature of these groups. So, for example, wherethe base wafer coat layer material is an isopropyl alcohol-basedmaterial (for example conventional solder-fluxing agent, such as aWF-series Sparkle flux from Senju Metal Industry Co., discussed below),an alcohol soluble UV absorber, such as a benzophenone structure havingfour hydroxy (—OH) groups attached to it, can be used. If however, twoof the hydroxy groups on the benzophenone are changed to methoxy groups(—OCH3), then alcohol solubility will be lost. From another perspective,if the particular base wafer coat material selected is soluble in aspecific solvent, then a UV absorbing additive should be selected thatis soluble in that same solvent. So for example, if the wafer coatformulation is soluble in toluene, then a toluene-soluble UV absorbingadditive should be used in the base wafer coat formulation.

As disclosed herein, the base wafer coat layer material is a solderfluxing agent. One of ordinary skill appreciates however that any numberof other materials suitable for protecting bumps during the laser scribeoperation can be used as the base wafer coat layer. After the additiveis included in the base wafer coat layer, the UV absorber-containingwafer coat layer can be applied via spray coating, dip coating, spincoating, screen printing, or the like.

In one specific implementation of this embodiment, UV additive Uvinul®3050 from BASF Aktiengesellschaft (2,2′,4,4′-Tetrahydroxybenzophenone)was added to a WF-series (WF-7700) Sparkle flux (base wafer coatmaterial) from Senju Metal Industry Co., Ltd. Uvinul® 3050 was includedin the wafer formulation in amounts that ranged from approximately 0.25to 1.0 wt % based on dry solids (e.g., amounts of Uvinul® 3050 rangingbetween 0.25 grams and 1.0 grams were added to 100 grams of Sparkleflux). The UV absorber-containing wafer coat formulation was then spraycoated onto a wafers at different thicknesses ranging from 15 and 25microns. Finally the wafers were laser scribed and then washed off withwater. In those wafers that included the UV modified wafer coating,residual scribe line defects were eliminated. This is in contrast tocontrol wafers that were coated using a conventional wafer coat processand then laser scribed. In these wafers, mild to severe defects remainedin the street (scribe) regions.

Of course, one of ordinary skill understands that the amount of UVabsorbing additives added to the wafer coat base layer is dependentupon, among other things, the efficiency of the UV absorber (i.e., itsability to absorb UV radiation) and the amount of UV absorptionrequired. So in cases where a UV absorbing additive is more or lessefficient than that of the Uvinul® 3050 described herein, then more orless additive may be used in the base wafer coat layer to achieve anamount of UV absorption desired. Similarly, to the extent that more orless additive (e.g., Uvinul® 3050) is added to the base wafer coat layerdescribed herein, then more or less UV absorption may occur as a resultof the formulation modification, as so desired.

The embodiments discussed with respect to FIGS. 5 and 6 facilitate theuse of UV lasers for performing laser scribing operations. This can beadvantageous when scribing narrow street regions because the UV lasershave relatively small ablation spot sizes. Also, the UV absorber can beadded to the wafer coat formulation in specific amounts that permittuning the degree of UV absorption/ablation called for by the process.The ability to vary the amount of UV absorber in the wafer coatformulation also provides an ability to adjust the wafer coat layer's UVabsorption independent from its thickness. This can be a considerationwith respect to throughput at the laser scribing operation (i.e. theamount of UV absorber in thick wafer coat layers can bereduced/increased to improve laser scribing cycle time). In addition, byusing a UV absorber, the wafer coat layer can remain transparent tovisible light. This allows the laser scribe alignment vision system toidentify and use fiducial marks to align the wafers. The UVabsorber-containing wafer coat layer can also be incorporated intoexisting manufacturing lines without the need to make significantequipment and/or process flow changes. Moreover, while the forgoingdiscussion focused on the use of UV absorbers in conjunction with wafercoat formulations to enhance ablation of the wafer coat layer by lasersthat operate in the UV portion of the electromagnetic spectrum, one ofordinary skill recognizes that variations of these embodiments cansimilarly be used to enhance the absorption of other types of lasers(e.g., infrared (IR) lasers) that are used to scribe wafers.

In accordance with a final alternative embodiment, discussed withrespect to FIGS. 7 and 8, a conformal wafer coat layer is disclosed. Inthis embodiment, problems associated with wafer coat layernon-uniformity are addressed by depositing a conformal wafer coat layer707 over bumps 706 and device layers 704, as shown in FIG. 7. Like theconventional process, after laser scribe, the conformal wafer coat layer(and laser scribing debris) is removed using a process that iscompatible with the wafer coat layer.

The conformal wafer coat layer disclosed herein overcomes wafer-coatlayer thickness variability problems encountered with conventional wafercoat layers. These variability problems are primarily due to the liquidnature of the wafer coat deposition process. For example, wafer coatlayers that use a BGA attach flux (e.g., a WF series Sparkle flux) asthe wafer coat material are typically sprayed-on, spun-on, dipped-on orscreen printed on.

All process where the flux is in liquid form when applied to the wafersurface. This means that the resulting wafer coat layer thickness andconformality can be affected by many factors, such as underlying surfacegeometry variations, wafer coat layer thickness, surface tensionvariations between the wafer coat layer and different types ofunderlying structures, drying time, operator handling, etc.

Typically, adequate bump protection is only achieved by forming asufficient thickness of wafer coat material over the bumps. In manycases, this means that an excessive wafer coat layer thickness is formedin scribe line regions. As shown in FIG. 8, the wafer coat layer 807must be deposited at least to a minimum thickness 812 that will coverthe bumps 806 and adequately protect them from debris (not shown).However, this often times means, due to the wet nature of the depositionprocess as discussed previously, that the thickness 810 of the wafercoat layer in regions overlying the device layers 804 can be muchthicker. For example, in one case where a spray on process was used todeposit a conventional BGA flux wafer coat layer, the thickness 812 ofthe wafer coat layer over the bumps measured 1-2 microns while thethickness 810 of the wafer coat layer in regions over the scribe linemeasured greater than forty microns. Since the laser has to ablate thedevice layer in regions where the wafer coat layer is for all intensivepurposes it's thickest, the extra thickness has the potential tointerfere with the laser's functioning and performance. For example, theadded thickness means more wafer coat layer material is now available tocontaminate the laser's lens, which can contribute to laser scribingsystem down-time. Also, to the extent that a thicker wafer coat layertakes longer to remove than a thinner one, the excess thicknesscontributes to increased cycle time during the laser scribing process.Moreover, in so far as the laser beam 808 encounters excessive and/ornon-uniform wafer coat layer thicknesses near the laser scribe lines(for example if the laser were to scribe through the scribe line inregions where the wafer coat layer thickness was 814 as opposed to 810)additional/unanticipated optical refraction, diffraction, and scatteringof the laser beam could result. This can produce shallow and/or evendiscontinuous laser scribing thereby contributing to the formationunablated scribe line material defects. It is worth noting here thatdefectivity levels can be much more pronounced in wafers where thesurface topology includes sudden and significant depth changes such asin cases where the wafers already have deep trenches in the streetregions.

In accordance with embodiments discussed with respect to FIG. 7, theforegoing problems can be reduced by depositing a conformal wafer coatlayer over the bumps prior to the laser scribe operation. As shown inthe cross-section 700 of FIG. 7, the conformal wafer coat layerdeposition process produces a wafer coat layer 707 that has a thickness712 over the bumps 706 that is relatively close to the thickness 710over the scribe line region. In this way, problems encountered by thelaser beam 708 with excessive wafer coat layer thicknesses are reducedand/or avoided. For the purpose of this specification, relatively closemeans that the thickness 712 of the wafer coat layer material on thebump at is within approximately twenty-five percent of the thickness ofthe wafer coat layer at a point 710 adjacent the bump. So for example,if the thickness 710 of the wafer coat layer laterally adjacent thesidewall 714 of bump 706A is 100 nanometers, then the thickness 712 ofthe wafer coat layer on the bump 706A, because it varies by less than25%, would be in a range of approximately 75-125 nanometers.

In accordance with one implementation of this embodiment, the conformalwafer coat layer can be deposited in a physical vapor deposition (PVD)chamber by heating a PVD source at a temperature and pressure capable offorming the conformal layer over the wafer. Then, after wafer scribingis complete, an appropriate solvent or evaporation process can be usedto remove the conformal layer.

In one specific implementation, wafer(s) are put inside a PVD chamberthat contains a di-para-xylylene dimer source material. The chamber ispumped down to a pressure and the source is heated to a temperaturecapable of subliming the dimer so it can deposit onto the surface of thewafer over the bumps. In accordance with one embodiment, the pressure isin a range of approximately 0.75-1.25 Torr and the temperature is in arange of approximately 130-170 degrees Celsius.

After forming the conformal wafer coat layer, the wafer(s) are removedfrom the PVD chamber and then laser scribed. Following the laserscribing operation, the scribing debris and the di-para-xylylene layercan be cleaned off using a non-polar solvent, such as acetone. Thedi-para-xylylene is soluble in the non-polar solvent and can thereforebe cleaned off easily.

The conformally deposited wafer coat layer disclosed herein is formedusing a relative dry, solvent free process with no liquid phase. Ittherefore does not pool, bridge, or exhibit meniscus properties duringformation, and it can provide a uniform, pin-hole free, and conformalsurface coverage on top of the wafers. The thickness of the film can beoptimized solely based on its ability to provide sufficient bumpprotection during the laser scribing process. In addition, because thiswafer coat layer process has the potential to provide improved conformalcoverage independent of surface topology, the process recipe can bestandardized on wafers having different devices, die sizes, etc.

Removal of the di-para-xylylene thin film (and the laser generateddebris) can alternatively be removed by placing the wafer(s) back to avacuum chamber, pressurizing and heating the chamber to createconditions that promote vaporization of the di-para-xylylene thin filmfrom the wafer surface. In one specific embodiment, similar to thedeposition process, the chamber is heated to a temperature in a range of130-170 degrees Celsius and the chamber pressure is in a range ofapproximately 0.75-1.25 Torr. Under these conditions, the coateddi-para-xylylene layer can be vaporized from the wafer surface. Thisadditionally loosens laser scribing debris deposited on the conformalwafer coat layer. The loosened laser scribe debris can then be washedoff using a water rinse process.

The embodiments discussed with respect to FIGS. 7 can provide a uniformand conformal coating on a wafer independent of its surface geometryand/or street profile. This facilitates laser scribing of wafers withhighly complex surface structures and trenched streets. In addition, thecoating process can be optimized for one product type and then easily betransferred to other types of wafers with minimal changes to theprocess. This simplifies the engineering design and related supportprocesses required to integrate the conformal wafer coat layer process.Furthermore, the conformal wafer coat layer disclosed herein helps tofurther efforts to reduce die sizes because the need to position bumpsaway from scribe line streets by certain distances to compensate forwafer coat layer variation is reduced and/or eliminated. Thisfacilitates die size reduction by allowing the die bumps to be placedcloser to the edge of the die. This translates to an increase in thenumber of die per wafer and thereby reduces fabrication and assembly perunit costs.

In the foregoing embodiments, various wafer coat layer formulations andprocesses have been disclosed that can improve laser scribingmanufacturability and yields. One of ordinary skill recognizes thatthese formulations and processes are not mutually exclusive from eachother and that they can be combined to collectively take advantage oftheir individual benefits. For example the OSP additive and the UVabsorber additive can be included together as part of a single wafercoat formulation. Similarly, an OSP and/or an UV absorber sources canalternatively be included in the PVD chamber along with the conformalwafer coat layer source to form a conformal wafer coat layer thatadditionally protects bumps against oxidation and/or absorbs UV energyfrom the laser.

The various implementations described above have been presented by wayof example only and not limitation. Having thus described in detailembodiments of the present invention, it is understood that theinvention defined by the appended claims is not to be limited byparticular details set forth in the above description, as many apparentvariations thereof are possible without departing from the spirit orscope thereof.

1. A method for processing a semiconductor substrate comprising:depositing a wafer coat formulation that includes an oxidationinhibiting additive over conductive structures on the semiconductorsubstrate, wherein depositing forms an oxidation prevention layer overthe conductive structures and a wafer coat layer over the semiconductorsubstrate; removing the wafer coat layer; and attaching thesemiconductor substrate to a packaging substrate.
 2. The method of claim1 further comprising laser scribing the semiconductor substrate betweendepositing and attaching.
 3. The method of claim 2, wherein theoxidation inhibiting additive is further characterized an organicsurface protectant additive and the conductive structures are furthercharacterized as conductive bumps, wherein the organic surfaceprotectant additive facilitates formation of the oxidation preventionlayer, and wherein the oxidation prevention layer reduces bump oxidationbetween depositing and attaching.
 4. The method of claim 3 furthercomprising removing the organic surface protectant incident toattaching.
 5. The method of claim 3, wherein the wafer coat layerincludes a BGA attach flux material.
 6. The method of claim 3, whereinthe organic surface protectant includes at least one of benzotriazol,benzimidazol, and imidazol.
 7. The method of claim 3, further comprisingremoving portions of the wafer coat layer using a water-based solution.8. The method of claim 1, wherein depositing is further characterized asincluding a process selected from the group consisting of a spray-onprocess, a spin-on process, dipping, and screen printing.
 9. The methodof claim 3, wherein the attaching is further characterized as includinga capillary underfill process.
 10. The method of claim 3, wherein theattaching is further characterized as including a no-flow underfillprocess.
 11. A method for processing a semiconductor substratecomprising: depositing a wafer coat formulation that includes anradiation absorber additive over a semiconductor substrate, wherein theradiation absorber additive enhances a wafer coat layer's sensitivity toablation during a laser scribing process; and laser scribing thesemiconductor substrate.
 12. The method of claim 11, wherein a laserused to scribe the wafer is further characterized as an ultravioletlaser and wherein the radiation absorber additive promotes absorption ofultraviolet radiation from the ultraviolet laser.
 13. The method ofclaim 12, further comprising removing the wafer coat layer via ablationwhile laser scribing the semiconductor substrate.
 14. The method ofclaim 12, further comprising degrading the wafer coat layer via ablationwhile laser scribing the semiconductor substrate.
 15. The method ofclaim 12, further comprising burning away portions of the wafer coatlayer from the top down while laser scribing the semiconductorsubstrate.
 16. The method of claim 12, further comprising ablating andremoving portions of the wafer coat layer prior to ablating and removingan entire thickness of portions device layers below the portions of thewafer coat layer.
 17. The method of claim 12, wherein the radiationabsorber additive is added in an amount that increases absorption of thelaser's UV radiation without causing substantial ablation of the wafercoat layer.
 18. The method of claim 12, wherein the radiation absorberadditive includes an additive selected from the group consisting ofbenzophenones, diphenyl acrylates, cinnamates, and sterically hinderedamines.
 19. The method of claim 12, wherein the wafer coat formulationincludes a fluxing agent and wherein the radiation absorber additiveincludes tetrahydroxybenzophenone.
 20. The method of claim 19, whereinthe tetrahydroxybenzophenone is further characterized as2,2′,4,4′-Tetrahydroxybenzophenone that is included in the wafer coatlayer formulation in an amount ranging from approximately 0.25 to 1.0 wt%.
 21. A method of processing a semiconductor substrate comprisingdepositing a wafer coat layer over a semiconductor substrate, wherein athickness of portions of the wafer coat layer overlying a conductivebump is within twenty-five percent of a thickness of portions of thewafer coat layer adjacent the conductive bump.
 22. The method of claim21, wherein the wafer coat layer is deposited in a physical vapordeposition chamber by heating a physical vapor deposition source at atemperature and a pressure capable forming the conformal layer over thesemiconductor substrate.
 23. A method for forming a conformal wafer coatlayer over a semiconductor wafer comprising: placing a semiconductorsubstrate into a physical vapor deposition chamber; heating a physicalvapor deposition source at a temperature and a pressure capable offorming the conformal layer over the semiconductor wafer; depositing theconformal wafer coat layer onto the semiconductor wafer; and removingthe semiconductor wafer from the physical vapor deposition chamber;laser scribing the semiconductor wafer; and removing the conformal wafercoat layer.
 24. The method of claim 23, wherein the physical vapordeposition source is further characterized as a di-para-xylylene dimersource material.
 25. The method of claim 24, wherein the temperature isin a range of approximately 130-170 degrees Celsius.
 26. The method ofclaim 25, wherein the pressure is in a range of approximately 0.75-1.25Torr.
 27. The method of claim 26, wherein removing the conformal wafercoat layer includes applying a non-polar solvent to the conformal wafercoat layer.
 28. The method of claim 27, wherein the non-polar solvent isfurther characterized as acetone.
 29. The method of claim 26, whereinremoving the conformal wafer coat layer includes placing thesemiconductor wafer back into a vacuum chamber and then heating andpressurizing the vacuum chamber to create conditions that promotevaporization of the conformal wafer coat layer from the wafer surface.30. The method of claim 29, wherein the conditions that promotevaporization are further characterized as heating the vacuum chamber toa temperature in a range of approximately 130-170 degrees Celsius andpressurizing the vacuum chamber to a pressure in a range of 0.75-1.25Torr.